Relation between the sliding friction angle of rock joints and the friction angle of intact cores at the brittle-ductile transition: An experimental study

The sliding friction angle is a fundamental mechanical property both for intact and jointed rocks. The aim of this paper is to compare the value of sliding friction angle obtained from direct shear tests on rock fractures and that obtained from triaxial compression tests on intact rock cores. For this reason, 286 direct shear tests were carried out on artificial rough tension fractures of ten different rock types under normal stresses 5 kPa - 2 MPa, using the multistage shear procedure with repositioning of the joint before each shearing. Another 112 conventional triaxial compression and unconfined compression tests were carried out on NX size (54 mm) intact specimens from the same rock types under various confining pressures up to 70 MPa. The sliding friction angle from the direct shear tests was determined from the measured peak friction angle and the corresponding dilation angle and that from the triaxial compression tests from the stress state at the estimated brittle-ductile transition. The results show that the values of the sliding friction angle of the studied rocks determined by the two methods are approximately equal (differences < 1 degrees) and agree very well with well-established experimental data for similar types of rocks reported in the literature. They ranged between 32.5 degrees and 40.2 degrees with those of the weaker and more ductile carbonate rocks to be distinctly higher (35.4 degrees-40.2 degrees) than those of the stronger and more brittle silicate ones (range 32.5 degrees-35.6 degrees). The average transition principal stress ratio was 5.7 for carbonate and 4.1 for silicate rocks.